The present invention relates generally to strain relief terminations for electrical cables and more particulary to a crimpless strain relief termination for a coaxial cable that maintains constant impedance characteristics across the strain relief termination.
Strain relief terminations for coaxial cables are important components for electrical measurement probes where the coaxial cable connects the probe head of the probe to a probe termination box. One such strain relief termination is described in U.S. Pat. No. 5,061,892, titled xe2x80x9cElectrical Test Probe Having Integral Strain Relief and Ground Connectionxe2x80x9d, and assigned to the assignee of the present invention. A strain relief termination has a tubular shaped member and a flat surface portion that transitions from the tubular member. The flat surface portion is secured to a substrate in the measurement probe head. The outer insulating material of the coaxial cable is removed and the outer conductive shielding material is folded back over the outer insulating material and positioned in the tubular member of the strain relief termination. The substrate and strain relief termination are inserted into an electrically conductive elongate body and the conductive body is crimped at the location of the tubular member of the strain relief termination using an appropriate crimping tool to capture and secure the coaxial cable within the electrically conductive elongate body.
A major drawback to this and other similar types of designs is that the coaxial cable or transmission cable is crimped, in part, to provide the strain relief and pull-strength on the cable. Such crimping causes changes in the characteristic impedance of the coaxial cable or transmission cable at the crimping location. The impedance changes in the coaxial cable adversely affects the overall bandwidth characteristics of the measurement test probe.
U.S. patent application Ser. No. 09/300,980, filed Apr. 28, 1999, entitle xe2x80x9cStrain Relief, Pull Strength Termination with Controlled Impedance for an Electrical Cablexe2x80x9d and assigned to the assignee of the present invention, describes a termination having a carrier with a flat portion and a tab portion extending from the flat portion. A transmission cable, such as a coaxial cable that has a portion of its outer insulating layer removed to expose the outer shielding conductor, is positioned on the flat portion the carrier. A securing means, such as an adhesive or solder, secures the cable to the carrier with a controlled impedance. A housing receives the carrier and has a tab formed in the housing that is movable from a first to a second position with the housing tab engaging the carrier tab in the second position. The combination of the housing tab engaging the carrier tab and the securing of the cable to the carrier with solder or adhesive provides the strain relief termination for the coaxial cable with controlled impedance.
While the above described strain relief termination works well for providing strain relief termination for the measurement probe head, it is not well suited for providing strain relief at the termination box. What is needed is the crimpless strain relief termination for coaxial cable that maintains a controlled impedance across the strain relief termination. It should be adapted to function with measurement probe termination boxes. The crimpless strain relief termination should also be inexpensive to manufacture and easy to assemble.
Accordingly, the present invention is to a crimpless strain relief termination for a coaxial cable and its method of manufacture. The crimpless strain relief termination has a crimpless mechanical termination having an electrically conductive bushing with a bore there through and a knurled region on the outer surface of the bushing. The bushing is positioned over an end portion of the coaxial cable that has its outer insulating layer removed and adjacent to the shielding conductor. The shielding conductor folded over the bushing to position a portion of the shielding conductor adjacent to the knurled region. A tubular shaped heat shrinkable material having an inner surface covered with an adhesive is positioned on the coaxial cable over the electrically conductive bushing. The heat shrinkable material is heated such that the adhesive is activated and the material shrinks to capture the shielding conductor between the heat shrinkable material and the electrically conductive bushing. The crimpless mechanical termination is positioned in a strain relief bushing having a bore there through with one end of the bore having an inwardly formed shoulder that engages the crimpless mechanical termination to provide mechanical strain relief for the coaxial cable.
Preferably, the strain relief bushing has threads formed on its outer surface and a outwardly extending flange disposed approximate to the threaded outer surface. A threaded nut is provided that is positioned on the threaded outer surface of the strain relief bushing that captures a support member between the flange and the threaded nut. The strain relief bushing may be formed of an electrically conductive material to provide an electrical connection between the outer shielding conductor of the coaxial cable to the support member.
The method of forming a crimpless strain relief termination for the coaxial cable includes the steps forming a crimpless mechanical termination by positioning the electrically conductive bushing over the end portion of the coaxial cable with a portion of the shielding conductor folded over the electrically conductive bushing to position the shielding conductor adjacent to the knurled region of the bushing. The tubular shaped heat shrinkable material is positioned over the electrically conductive bushing and the material is heated to activate the adhesive and shrink the material to capture the shielding conductor between the heat shrinkable material and the electrically conductive bushing. The coaxial cable is positioned through a strain relief bushing such that the inwardly formed shoulder engages the crimpless mechanical termination to provide mechanical strain relief for the coaxial cable.
The positioning of the coaxial cable step includes the additional step of positioning the strain relief bushing through an aperture formed in a support member such that an outwardly extending flange on the strain relief bushing engages the support member. The positioning of the strain relief bushing further includes the steps of engaging a threaded nut on threads formed on the outer surface of the strain relief bushing, and tightening the treaded nut on the strain relief bushing to capture the support member between the nut and the outwardly extending flange.
The objects, advantages and novel features of the present invention are apparent from the following detailed description when read in conjunction with appended claims and attached drawings.